That's a good strenuous walk Hugo, shame about the weather! I hope your mate's knee is OK. I took a fall on wet ground on the final section of Offa's Dyke above Prestatyn a few months ago and I'm still restricted by a pulled thigh/hip muscle and knee strain. It always seems to happen on the descent, mind you I've never fancied going down Pen-yr-Ole Wen ever, even in the dry! A good few years ago on a Good Friday a group of us got a lift to Ogwen Cottage and set off straight up Pen-yr-Ole-Wen, on up to Carnedd Dafydd, Carnedd Llewellyn and then down the Carneddau ridge to Drum, Bwlch-y-Ddeufaen and then round Tal-y-Fan and on over Pensychnant and Conwy Mountain and finally finished up in the old "Farmer's Arms" in Deganwy. We decided (in the "Farmer's" that night!), that we would do it on Good Friday every year but it never happened for some reason!
Another marathon we used to do when we were younger (and fitter!) was to leave the cars at Pen-y-Gaer above Llanbedr-y-Cennin and then go up to Carnedd Llewellyn via Drum and Foel Fras, down across Craig-yr-Ysfa, up over Pen-yr-Helgu Du and Pen Llithrig-y-Wrach and then down past the old quarry workings at Cedryn and back along the floor of Cwm Eigiau past the burst dam and back over to the start at Pen-y-Gaer.
Those were the days!